In order to produce a high output power, semiconductor lasers must be operated either significantly above the lasing threshold or employ a large active area that produces gain. In both circumstances, heat dissipation from Joule heating becomes an overriding concern since, generally, the laser efficiency decreases substantially as the temperature of the active region increases. It is therefore beneficial to position a heat sink comprising a material with relatively high thermal conductivity (such as a noble metal or diamond) as close as possible to the epitaxial surface of the laser so that it can spread the heat away from the active region, minimizing the temperature increase.
A laser apparatus can be mounted to a heat sink either epitaxial-side-up (“epi-up”), with the substrate side of the laser apparatus mounted on the heat sink, or epitaxial-side-down (“epi-down”), with the active epitaxial side of the laser chip mounted directly to the heat sink.
FIGS. 1A and 1B illustrate exemplary configurations of conventional epi-up ridge waveguide lasers in accordance with the prior art. As shown in FIGS. 1A and 1B, a typical semiconductor laser includes p(+) top optical clad/Active Gain/n(−) bottom optical clad layers 101/102/103, respectively. These layers, which constitute the laser portion of the chip, are grown on a substrate 104 and are patterned into a laser ridge 120. In the exemplary devices shown in FIGS. 1A, 1B, and 1C, the n(+) region is closest to the substrate, though in other configurations, the p(+) region may be closest.
In an epi-up configuration, the substrate is secured to a high thermal conductivity heat sink 105. As shown in FIG. 1A, heat from the laser (i.e., layers 101-103) flows through substrate 104 and the solder 106 and dissipates into heat sink 105. However, epi-up mounting of the laser chip does not provide the most efficient dissipation of the generated heat. This is because the heat dissipation path that must traverse the substrate tends to have a high thermal resistance, since the distance to the heat sink is quite long and the substrate thermal conductivity is generally much lower than that of the heat sink material.
The heat dissipation in an epi-up ridge laser configuration can be improved somewhat by a layer of heat-spreading material 107, typically a 3-5 μm thick layer of electroplated gold (e-Au), deposited on top of laser ridge 120, as shown in FIG. 1B. Since only the top cladding 101 separates the gold from the active region, the heat will diffuse (primarily vertically, but also to some extent laterally for narrow ridges) toward and into the electroplated layer 107, although it must still traverse the substrate 104 before it reaches heat sink 105. While this approach can significantly reduce the thermal resistance as compared to epi-up mounting without the gold electroplating, it is still far from ideal since the thin layer of gold has far less capacity to dissipate the heat than a heat sink comprising a thick layer of heat-spreading material.
FIG. 1C illustrates an exemplary epi-down mounting arrangement. As shown in FIG. 1C, in an epi-down mounting arrangement the laser 101-103 side of the chip is mounted directly to the heat sink 105, with the substrate 104 being “on top.” Such a mounting configuration, which may also optionally incorporate an e-Au layer between the laser and the heat sink, is much more effective to dissipate heat because it places the heat sink 105 in the immediate vicinity of the laser's active region. Epi-down mounting decreases heating in the active gain regions 102 of both narrow and wide laser ridges, and is especially useful for lasers having wide ridges because the heat conduction is mostly in the vertical direction.
As illustrated in FIG. 1C, to maintain mechanical stability in an epi-down mounting configuration, the laser ridge is typically defined by narrow trenches 108 etched in the chip, with laser facets 110 defining the longitudinal dimensions of the trenches. The laser apparatus is typically secured to the heat sink by means of indium or gold-tin alloy solders which form a thermal bond between the laser and the heat sink. However, when pressure is applied to secure a thin, uniform solder joint, any excess molten solder must flow out of the region between the two bonded layers, collecting as solder reform 106 shown in FIGS. 1A/1B/1C. This escaping solder often ends up on the laser facets or on the sidewalls of the substrate, thereby blocking the light emission or shorting the device, e.g., via shorting paths 109a/109b/109c shown in FIG. 1C, because the solder is electrically conductive.
One approach to control the flow of the solder is to pattern the heat sink with relief areas where the escaping solder may accumulate. While this may sometimes be effective, it suffers from the impracticality of having to pattern the heat sink to conform to a particular laser chip size and geometry. Because it can be extremely challenging to fully control the disposition of the flowing solder, even in mature diode laser packaging facilities the yield of good devices is often degraded by shorting, blocking, or inadequate thermal bonding in some fraction of the mounted lasers. At the other extreme, personnel who have not developed the considerable expertise required for effective epi-down mounting may experience very low or even zero yields.
The present invention solves these problems by providing, with high yield, an improved epi-down structure which relieves the pressure on the solder.